Drivers: hv: kvp: Use MAX_ADAPTER_ID_SIZE for translating adapter id

There's a bug which passes the output buffer size as MAX_IP_ADDR_SIZE,
when converting the adapter_id field to UTF16. This is much larger than
the actual size (MAX_ADAPTER_ID_SIZE). Fix this by passing the proper
size.

Fortunately, the translation is limited by the length of the input. This
explains why we haven't seen output buffer overflow conditions.

Signed-off-by: Alex Ng <alexng@messages.microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>